In a large variety of applications it is desirable to be able to control the intensity of light emitted by an electrically driven light source. The present invention concerns especially the user interfaces of portable electronic apparatuses, where artificial illumination is used to enhance the usability of the user interface when ambient light is not enough, and to increase visual attractiveness. Typical illuminated user interface components include but are not limited to displays and keypads. Light sources are typically either discharge tubes or LEDs (Light Emitting Diodes).
FIG. 1 illustrates a known principle of providing controllable illumination to a user interface. A light source 101 is coupled between the output of a lighting controller 102 and ground. The light source 101 conceptually represents any arrangement of one or several physical light-emitting devices. The lighting controller receives a constant operational voltage Vcc from a voltage source 103, and lighting control commands from a microprocessor 104. A sensor 105 is coupled to an input of the microprocessor 104. The task of the sensor 105 is to detect the need for user interface illumination. It provides a measurement result to the microprocessor 104, which translates the measurement result into a lighting control command and outputs it to the lighting controller 102. The lighting controller 102 controls the voltage and/or current going to the light source 101. The sensor 105 may be e.g. a phototransistor that measures the amount of ambient light. Alternatively the sensor 105 may exist only “conceptually” in a software routine executed by the microprocessor 104: the software routine may e.g. dictate that the occurrence of an incoming call must be responded to by changing the illumination of the user interface in a certain way.
The most basic form of illumination control involves only setting lights on or off according to need. More sophisticated lighting control arrangements are capable of providing several levels of illumination intensities. FIG. 2 illustrates schematically two known ways of obtaining different illumination intensities with LED sources. The topmost graph 201 represents the principle of varying the electric current fed into the LED(s). The middle graph 202 illustrates the principle of pulse width modulation (PWM), in which the current fed into the LED(s) is repeatedly switched between zero and a constant non-zero value. The duty cycle, i.e. the length in time of the ON pulse compared to the combined length of consecutive ON and OFF periods, is varied according to the desired light intensity. In the drawing the duty cycle is first 80%, then 40%, then 20% and finally 60%. Graph 203 at the bottom shows how both of the above-mentioned methods result in a varying intensity of light emitted by the LED(s).
Known prior art publications that tackle the problem of providing variable output intensities include DE 19 71 1885, DE 19 81 4745 and U.S. 2003/043611 A1. Of these, the last-mentioned presents an interesting embodiment in which the duty cycle of a PWM controller is kept essentially constant at 50%, but the switching frequency is varied in relatively wide limits like between 200 kHz and 1 MHz. In addition to a light source there is a resonant element coupled to the output of the PWM controller. The resonance characteristics of the combined output circuit cause the light source to emit light at a highest intensity level when the switching frequency coincides with the resonance frequency of the output circuit. The farther the switching frequency goes from the resonance frequency, the lower is the intensity of emitted light.
The drawbacks of the prior art arrangements become apparent when a question is raised about the number of different intensity levels that can be obtained. Even if the theoretical principle of current control or pulse width modulation could enable even a stepless control between zero and a maximum value, practical current controllers and PWM controllers that are available for integration with other electronic functionalities of a portable electronic device usually have a relatively modest number of possible output modes. A typical integrated PWM controller circuit includes three or four control switches or single-bit control input lines, the states of which affect the duty cycle (or the switching frequency in the case of U.S. 2003/043611 A1). Consequently there are only 8 or 16 possible intensity levels of emitted light. These may well be enough for providing a number of steady-state conditions to choose from, but they are certainly not sufficient to implement changes of intensity that a human user should perceive as stepless dimming or brightening.